Small-scale convection in a plume-fed low-viscosity layer beneath a moving plate

Abstract

Two-dimensional simulations using a thermomechanical model based on a finite-differencemethod on a staggered grid and a marker in cell method are performed to study the plume-lithosphere interaction beneath moving plates. The plate and the convective mantle are modelledas a homogeneous peridotite with a Newtonian temperature- and pressure-dependentviscosity. A constant velocity, ranging from 5 to 12.5 cmyr;-1, is imposed at the top of theplate. Plumes are generated by imposing a thermal anomaly of 150 to 350K on a 50 km widedomain at the base of the model (700 km depth); the plate atop this thermal anomaly is 40Myrold. We analyse (1) the kinematics of the plume as it impacts the moving plate, (2) the dynamicsof time-dependent small-scale convection (SSC) instabilities developing in the low-viscositylayer formed by spreading of hot plume material at the base of the lithosphere and (3) theresulting thermal rejuvenation of the lithosphere. The spreading of the plume material at thebase of the lithosphere, characterized by the ratio between the maximum down- and upstreamhorizontal (dimensionless) velocities in the plume-fed sublithospheric layer, Peup/Pedown dependson the ratio between the maximum plume upwelling velocity and the plate velocity, Peplume/Peplate. For fast plate velocities and sluggish plumes (low Peplume/Peplate), plate motiondrags most plume material and downstream flow is dominant. As Peplume/Peplate increases, an increasing part of the plume material flows upstream. SSC systematically develops in theplume-fed sublithospheric layer, downstream from the plume. Onset time of SSC decreaseswith the Rayleigh number. For vigorous plumes, it does not depend on plate velocity. Formore sluggish plumes, however, variations in the plume spreading behaviour at the base ofthe lithosphere result in a decrease in the onset time of SSCs with increasing plate velocity. In any case, SSC results in uplift of the isotherm 1573K by up to 20 km relative to its initialequilibrium depth at the impact point.© The Authors 2013.